Rhodium–Carbon Bond Energies in Tp′Rh(CNneopentyl)(CH2X)H: Quantifying Stabilization Effects in M–C Bonds

Abstract

A series of substituted methyl derivatives of the type Tp′Rh(CNneopentyl)(CH₂X)H (CH₂X = CH₂C(═O)CH₃, CH₂C≡CCH₃, CH₂O-t-Bu, CH₂CF₃, CH₂F, CHF₂) was synthesized either by photolysis of Tp′Rh(CNneopentyl)(PhNCNneopentyl) in neat CH₃X or by exchange with the labile hydrocarbon in Tp′Rh(CNneopentyl)(n-pentyl)H or Tp′Rh(CNneopentyl)(CH₃)H. Only a single product was observed in each case. Clean reductive elimination was observed for all compounds in C₆D₆. Structures of these complexes and their corresponding chlorinated derivatives have been characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. Relative Rh–C bond energies are calculated using previously established kinetic techniques, and two separate linear correlations are observed versus known C–H bond strengths, one for the parent hydrocarbons, and one for the substituted hydrocarbons. Both correlations have slopes of 1.4, and are separated vertically by 7.5 kcal mol^–1 (−CH₂X above −C_xH_y). In addition, it is now clear that preferences for linear vs branched olefin insertion products in substituted derivatives can be predicted on the basis of the strengths of the β-C–H bonds. The DFT calculations of the metal–carbon bond strengths in these Rh–CH₂X derivatives with α-substitution show a trend that is in good agreement with the experimental results.